It is recently proposed in view of probable petroleum depletion to conduct gasification using, as raw material, petroleum coke as residue in petroleum refinery and/or any resource currently not efficiently utilized such as oilsand, bitumen, brown or other low-quality coal or other fossil fuel, biomass and tire chips to produce and efficiently utilize gasification gas comprising hydrogen, hydrocarbon and the like.
FIG. 1 schematically shows an example of a gasification system for production of gasification gas. Illustrated is a two-column gasification system with gasification and combustion furnaces 1 and 2, the gasification furnace 1 being fed with water vapor 3 from below to form a fluidized bed of bed material 4 (such as silica sand or limestone) for gasification of raw material (such as coal, biomass or plastic waste) charged to the fluidized bed, resultant gasification gas 6 being fed to a succeeding gas refining system (not shown).
The bed material 4 in the gasification furnace 1 is guided, together with unreacted char 7 produced in the furnace 1, through a duct 1a on the furnace 1 to a combustion furnace 2 in an overflow manner, is blown up by primary air 8 fed to the furnace 2 from below and is heated by burning of the char 7.
Combustion exhaust gas 9 blown up in the combustion furnace 2 together with the bed material 4 is then guided through a top of the furnace 2 to a cyclone 10 where the bed material 4 is separated. The separated bed material 4 is returned through a downcomer 11 to the gasification furnace 1 while the combustion exhaust gas 9 discharged through a top of the cyclone 10 is fed for treatment to a succeeding exhaust gas treatment system (not shown).
Specifically, the combustion exhaust gas 9 resulting from the char 7 burned in the combustion furnace 2, which contains nitrogen oxides (NOx) of the order of several hundred ppm (several g/kg-fuel), requires to be denitrated. To this end, conventionally the combustion exhaust gas 9 discharged through the top of the cyclone 10 is guided, after heat recovered therefrom, to a catalytic NOx removal device (not shown) with NOx removal catalyst where ammonia, urea or the like is used as reducing agent to reduce the nitrogen oxides (NOx) in the combustion exhaust gas 9 to harmless nitrogen (N2) and water vapor (H2O).
On the other hand, the gasification gas 6 produced in and discharged from the gasification furnace 1, which contains high levels of ammonia, passes thorough a tar reforming furnace 12 or the like and is fed to an ammonia recovery device 13 where the ammonia in the gasification gas 6 is separated and recovered.
Specifically, it has been known that several percent by weight or more of nitrogen is contained in coal fed as raw material 5 to the gasification furnace 1, even more nitrogen being contained in the case of biomass. As a result, in the gasification furnace 1 of the two-column gasification system shown in FIG. 1 where the water vapor 3 is used as gasifying agent, the gasification of the nitrogen-containing raw material 5 as mentioned in the above brings about the gasification gas 6 having high levels of ammonia.
The ammonia recovery device 13 comprises an ammonia absorber 14 which absorbs and removes ammonia through contact of the gasification gas 6 with an absorptive liquid such as water and an ammonia diffuser 16 which diffuses the ammonia as ammonia off-gas 17 through heating of the ammonia-absorbing absorptive liquid by feed of water vapor 15. The ammonia off-gas 17, which has high levels of ammonia and problematically ammonical, is guided to a treatment device (not shown) for reduction and release to atmosphere through catalytic decomposition into harmless nitrogen (N2) and water vapor (H2O).
Prior art literatures on gasification systems pertinent to the invention are, for example, Patent Literatures 1 and 2.